1. Field of the Invention
The present invention relates to a tool for mounting a reinforcing sleeve within a tube by hardrolling or expanding a portion of the sleeve into engagement with the tube walls. It is particularly useful in mounting sleeves within the heat exchanger tubes of a nuclear steam generator which are located in the peripheral regions of the steam generator channelhead where minimum clearance exists.
2. Description of the Prior Art
Tools for expanding a sleeve within a steam generator heat exchanger tube to form a leak limiting mechanical joint are generally known in the prior art. Such tools include a rotatable roller cage which holds a plurality of radially extendible rollers captured within pitched slots, and a rotatable tapered mandrel which is slidably movable within a centrally disposed bore in the cage. The roller cage is rotatably mounted to a long outer shaft which houses a rotatable inner shaft that is connected to the tapered mandrel at the end nearest the roller cage. A drive means is also provided to transmit rotational torque to the inner shaft and thus the tapered mandrel, as well as to axially move the tapered mandrel into engagement with the axially extendible rollers captured within the pitched slots of the roller cage.
In operation, the reinforcing sleeve is first inserted within the tube to be repaired and axially slided into a position where the ends of the sleeve bridge the damaged portion of the tube. Next, the roller cage of the expansion tool is inserted into the open end of the tube and inside of the sleeve to be expanded, and positioned adjacent to one end of the sleeve. At this juncture, the tapered mandrel is applying no radial force on the rollers captured within the cage, so the sleeve walls push the rollers into a radially retracted position within the slots in the roller cage. Next, the tapered mandrel is simultaneously rotated and biased toward the interior of the tube. The resulting axial movement of the tapered mandrel causes it to engage and to radially extend the rollers (which are somewhat loosely held within tapered slots within the cage body) until they engage the interior surface of the sleeve to be expanded. Once the rollers are radially extended into such engagement, the outer surfaces of the rollers become simultaneously frictionally engaged between the inner walls of the sleeve, and the outer surface of the rotating tapered mandrel. Such frictional engagement allows the tapered mandrel to simultaneously rotate the rollers within their respective cage slots, and orbit the rollers around the axis of rotation of the mandrel. Because the slots which receive these rollers are pitched at a small angle relative to the axis of rotation in the mandrel, the rotation and the orbiting of the rollers draws the mandrel within the open end of the tube in what may be generally described as a screwing or feeding motion, which, in turn, radially extends the rollers further and expands the sleeve into engagement with the inner wall of the tube.
Such tools have been useful in the repair of heat exchanger tube s in nuclear steam generators. Unfortunately, the usefulness of such tools has been limited to only those heat exchanger tubes in the middle portion of the tubesheet of the generator which are easily reached and not interfered with by the bowl-shaped walls of the primary side of the generator. In order to more clearly understand the limitation on the above noted tools, some basic understanding of the structure and function of a nuclear steam generator is necessary.
Nuclear steam generators generally comprise a secondary side, a tube sheet, and a primary side which circulates water heated from a nuclear reactor. An example of such a generator is disclosed in U.S. Pat. No. 4,262,402 assigned to Westinghouse Electric Corporation, the assignee of the present invention. The secondary side of the generator includes a plurality of U-shaped tubes, as well as an inlet for admitting a flow of non-radioactive feed water. Both the inlet and outlet ends of the U-shaped tubes within the secondary side of the generator are mounted in a tube sheet which hydraulically separates the primary side of the generator from the secondary side. The primary side of the generator is bowl-shaped and includes a divider sheet which forms two quarter-sphere shaped channel heads that hydraulically isolate the inlet ends of the U-shaped tubes from the outlet ends. Hot, radioactive water flowing from the nuclear reactor is forceably pumped through the first side of the bowl-shaped primary side into the inlet ends of the U-shaped tubes. This hot, radioactive water flows through these inlets up through the tube sheet, and circulates around the U-shaped tubes which extend within the secondary side of the generator. The water transfers its heat through the walls of the U-shaped tubes to the non-radioactive feed water flowing through the secondary side of the generator, thereby converting this feed water into non-radioactive steam which, in turn, powers the turbines of an electric generator. After the water from the reactor circulates through the U-shaped tubes, it flows back through the tube sheet, through the outlets of the U-shaped tubes and into the outlet section of the bowl-shaped primary side where it is recirculated back to the nuclear reactor.
The walls of the U-shaped heat exchanger tubes in such a nuclear steam generator can suffer any number of different forms of corrosion, degradation, denting, stress corrosion cracking, intragranular attack, and pitting. In order to repair the walls of the heat exchange tubes that have been corroded, a sleeve is inserted into a steam generator tube in concentric relationship with the inner walls of the tube to bridge the corroded or damaged length of the tube. To effectively bridge the damaged area, the sleeve must be sealed at the upper and lower ends thereof to the tube to form a leak limiting mechanical joint. Various lengths of sleeves can be used depending upon the length of the defect as well as the distance of the defect above the tube sheet.
The sleeves must be inserted through the tube sheet from the primary side of the steam generator. Because the sleeves and the rolling tools which expand them are rigid, the length of the sleeves and tools which can be inserted into the heat exchanger tubes is limited by the clearance provided between the bowl-shaped walls of the primary side and the tube sheet. Such clearance is relatively small in the peripheral regions of the tube sheet where the bowl-shaped walls curve inwardly and immediately under the tubesheet. While a relatively short sleeve could be inserted into the peripherally located tubes, there is no prior art rolling mandrel known to applicants that could be used to effectively and conveniently seal the upper and lower ends of the sleeve to the tube due to the insufficient clearance within the channel head for such prior art sleeve rolling tools to perform the rolling and expanding functions.
Flexible drive shafts per se are well known. It is also known to use a flexible drive shaft on a steam generator for the insertion of a tool into the tube sheet in the peripheral region of the channel head. These known devices, however, are not concerned with tools utilizing the large torques required by a rolling tool, but instead are used in tools requiring minimal torques, such as a rotopeening apparatus. A known example of a flexible drive shaft and rotopeening apparatus is disclosed in U.S. Pat. No. 4,616,496 to Hawkins, also assigned to Westinghouse Electric Corporation, the assignee of the present invention. This apparatus includes a flexible driven inner shaft which is strong enough to transmit sufficient torque for the driven rotopeening elements, but which would be entirely insufficient to drive a hard rolling mechanism to expand and seal a sleeve. Note that the outer housing is also rotatably driven, but the outer housing is only rotated slightly relative to the inner shaft and is not used as a torque transfer member.
In order to effectively join a sleeve within a heat exchanger tube, a rolling tool is required which expands the tube beyond its elastic limit and seals the sleeve to the tube. Large torques are required for this purpose. In view of the limited diametrical space afforded by a typical steam generator tube, (which is typically only about 0.875 inches in diameter), a flexible drive shaft as is known would not be able to transmit sufficient torque to the mandrel of a rolling tool to expand a sleeve within the tube and seal the sleeve to the tube, thus adequately bridging a repair area in the steam generator tubing.
Clearly, there is a need for a rolling tool capable of expanding and sealing a sleeve to a peripherally located steam generator heat exchanger tube. Such a tool should also be simple, reliable, remotely controllable and compact for easy manipulation and use within the primary side of the generator.